ENTITY RELATIONSHIP DIAGRAM
Entity relationship diagram (ERD) adalah suatu model yang menwakili hubungan antar entitas berbentuk diagram. ERD tersusun dari 3 aspek utama yaitu :
-          Entitas
-          Relasi
-          Atribut

Entitas dapat diartikan sebagai benda (a thing). Relasi berarti hubungan antar entitas dalam dunia nyata. Sedangkan atribut dapat diartikan sebagai sesuatu yang melekat pada entitas (benda).
Contoh :
 




Relasi antara dosen dan mahasiswa berupa “Dosen mengajar mahasiswa”. Hubungan dinyatakan dalam dunia nyata, bukan hubungan yang tertera pada sistem. Relasi menggunakan verb (kata kerja).
 







-          Atomic atribute                : atribut paling dasar yang tidak bisa dipisah lagi, contoh : email.
-          Composite atribute         :atribut yang tersusun dari sejumlah kombinasi atau grup atribut.   Contoh           forename, middle initial, lastname.
-          Multivalue atribute         : atribut multinilai, berisi banyak nilai.
-          Derived atribute               : atribut turunan dari atribut lain. Contoh : IPS (indeks prestasi Semester) menghasilkan IPK (indeks prestasi kumulatif).





                                                                                                                                                                                multivalue
                                     composite


                                                     atomic
                                                                                                                                                                                      key

derived
key
ketika candidate key dipilih menjadi identifier unik, dia menjadi kunci utama untuk entitas.
Contoh : username dan NIM.
Strong entity      : memiliki satu atau lebih key.
Weak entity       : tidak memiliki key.

TANGGUNGAN
 

PEGAWAI
 
Oval: ID PEGAWAIContoh :








Tanpa id pegawai, maka tanggungan akan menjadi weak entity.
Dengan id pegawai, maka tanggungan akan menjadi strong entity.
Sebuah Relationship memiliki derajat (degree) berdasarkan jumlah entitas yang terhubung antara lain: Unary, Binary, Ternary dan Quaternary.
1.       Unary
Tipe hubungan ini berbarti hubungan yang terjadi antara dirinya sendiri dlam sebuah entitas; contohnya: entitas Pegawai yang memiliki Relationship Supervisor. Pegawai yang menjadi Supervisor juga berasal dar entitas yang sama yaitu Pegawai sedangkan entitas yang dituju juga pada entitas Pegawai.
~
2.       Binary
Tipe hubungan ini antara dua entitas; contohnya: entitas Pegawai dan entitas Kantor Cabang.
~
3.       Ternary
Tipe hubungan ini antara tiga entitas; contohnya :entitas Sales, Produk dan Pelanggan yang memiliki sebuah Relationship yang bernama Penjualan.


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